Dysregulation of serotonergic neurotransmission is an important contributing factor in many psychiatric disorders. Therefore, central serotonergic neurons are the primary targets for tricyclic antidepressants and serotonin reuptake inhibitors (SSRIs), as well as psychostimulants and hallucinogenic drugs (I. S. J. Bonasera, L. H. Tecott, Pharmacol. Ther. 88, 133 (2000); J. A. Gingrich, R. Hen, Psychopharmacol. 155, 1 (2001); D. L. Murphy et al., Genes Brain Behav. 2, 350 (2003)). Tryptophan hydroxylase (Tph1) has long been considered as the sole rate-limiting enzyme for the synthesis of serotonin. However, the recently cloned Tph2 is preferentially expressed in the brain, while Tph1 is mainly expressed in the periphery (D. J. Walther et al., Science 299, 76 (2003); M. Bader et al., PCT Application WO 2004/007704).